Ken Suto

Tunable terahertz wave generation in the 3- to 7-THz region from GaP

Following the generation of tunable terahertz waves from GaP in the 0.5- to 3-THz region, we extended the frequency region up to 7 THz, using an optical parametric oscillator and a YAG laser (1.064 μm). The tuning angle θin increased superlinearly in the 3- to 7-THz region, so that the total reflection took place at 5 THz, which was avoided by rotating the crystal relative to the incident optic axis. As a result, terahertz output peak powers of 100 mW at up to 5.6 THz and 3 mW at 7 THz were obtained, at pump and signal energies of 3 mJ, respectively.

Semiconductor Raman laser

The semiconductor Raman laser has been realized by using a GaP crystal. Pumping is made by a Q‐switched YAG laser operating at 1.064 μm. The round‐trip loss in the Fabry‐Perot resonator is 2% or less. The Raman scattering from LO phonons stimulates in the 〈100〉 direction, while the forward and backward Raman scattering from TO phonons stimulate in the 〈110〉 direction. This semiconductor Raman laser is promising as a semiconductor far‐infrared radiation source.

Frequency-tunable high-power terahertz wave generation from GaP

A frequency-tunable terahertz wave was generated from GaP crystals using an optical parametric oscillator as the pump source and a YAG laser (1.064 μm) as the signal source. By tuning the very small angle, θin, between the pump and signal light beam directions, tunable terahertz waves over the frequency range from 0.5 to 3 THz were obtained. The THz frequency changed almost linearly with the angle θin. The precise noncollinear phase matching condition is discussed. The pulsed peak power of the THz wave was as high as 480 mW at 1.3 THz.

Spectral measurement of terahertz vibrations of biomolecules using a GaP terahertz-wave generator with automatic scanning control

Having previously generated frequency-tunable terahertz (THz) waves from GaP, we constructed a THz spectral measurement system with an automatic scanning control. We demonstrate the use of this THz spectrometer for measuring the transmittance spectra of D-(+)-glucose, 2-deoxy-D-glucose, D-(−)-fructose, and D-(+)-sucrose in the 0.8 to 5.5 THz (25 to 185 cm−1) frequency region with a spectral resolution of 3.2 GHz (0.1 cm−1), using a pyroelectric detector operated at room temperature. Each crystalline saccharide had different spectral features.

Minority‐carrier lifetime measurements of efficient GaAlAs p‐n heterojunctions

High‐efficiency Ga1−xAlxAs p‐n heterojunctions have been fabricated by the temperature‐difference method (ηRT, 1–3% at 6650 A), and the deep levels which govern the efficiency and the lifetime have been studied by I‐V, C‐V, photocapacitance, impedance, and electroluminescence measurements. The lifetime obtained by the impedance measurement and the deep‐level distribution obtained by the C‐V measurement have a correlation. The lifetime for efficient diodes is as low as 1 μs at 80 K. The temperature dependence of the lifetime was also measured. Photocapacitance spectra show a threshold photon energy of 0.65 eV. Arsenic vacancies are proposed for these deep levels.

GaP THz wave generator and THz spectrometer using Cr:Forsterite lasers

We have developed a type of THz wave generator that uses Cr:Forsterite lasers as the pump and signal sources for difference frequency generation in GaP (Cr:F source system). We confirmed the generation of THz waves in the frequency range from 0.3to7.5THz, which is just similar to that obtained using the THz wave generator previously developed utilizing yttrium aluminum garnet and optical parametric oscillator (OPO) lasers (OPO source system). A peak output power of 100mW was obtained from 1.2to5THz when the power of the two input beams was 3mJ each, similar to the OPO source system. A wide measurable frequency range from below 0.6THz to over 6THz was obtained by using the Cr:F source system as the light source of a spectrometer, which has the merits of simple structure, easy maintenance, and low cost compared with the OPO source system. Although the linewidth of the Cr:F source system is greater than that of the OPO source system, the THz spectrometer still has sufficient resolution for measuring solids o...

Continuous-Wave Frequency-Tunable Terahertz-Wave Generation From GaP

Continuous-wave (CW) single-frequency terahertz (THz) waves were generated using difference-frequency generation via excitation of phonon-polaritons in GaP. The two pump sources were an external cavity laser diode (LD) and an LD-pumped Nd : YAG laser. The power density of the latter beam was enhanced by using a ytterbium-doped fiber amplifier. The two incident beams were focused to near the wavelength of THz waves. This optical alignment enabled us to generate frequency-tunable CW THz waves in the 0.69-2.74 THz range. With a fixed angle between the pump beams, we obtained a frequency bandwidth as large as 600 GHz

Characteristics of terahertz-wave generation from GaSe crystals

Terahertz (THz) waves from 0.3 to 4.9 THz were generated from GaSe by difference-wave generation using a YAG laser and an optical parametric oscillator. The spectral THz intensity distribution matched the spectrum of the THz absorption coefficient well, which suggested the effect of imperfections in the crystal. The fairly narrow beam profile and narrow line width, together with the simplicity of the collinear configuration, make GaSe an effective THz-wave generator. It is compared with a GaP THz-wave generator.

Semiconductor Raman amplifier for terahertz bandwidth optical communication

Semiconductor Raman amplifiers are useful for frequency selection in terahertz bandwidth and wavelength division multiplexing (WDM) systems with terabit capacity, as well as direct terabit optical communication systems. We have developed GaP-AlGaP Raman waveguides with micrometer-size cross sections. We have reduced residual optical loss of the waveguide by improvement of the fabrication process and realized a low-loss waveguide that is 10-mm long, which has a continuous wave (CW) Raman gain of 3.7 dB. Also, the time-gated amplification with 80-ps pulse pumping is performed and 20-dB gain is obtained. These performances are very suitable for light frequency selection in terahertz bandwidth and WDM optical communication systems.

Paramagnetic Resonance of Trapped Holes in ZnSe : Ge and ZnSe : Pb

Paramagnetic resonance measurements have been made for Ge and Pb in ZnSe at 77°K. Hyperfine structures (HFS) arising from Ge 73 and from Pb 207 , and superhyperfine structures (SHFS) arising from Se 77 are observed. The large Se 77 superhyperfine splittings show that the wave functions for the EPR centers are localized on the surrounding Se 2- ions. This indicates that the resonances are due to holes trapped at the surrounding Se 2- ions. The EPR spectra are photo-sensitive. The photo-response of the signals shows that the trap levels are located deep in the forbidden energy gap.